Thermodynamics of Equilibrium in the Ultracentrifuge

An exact equation has been derived which can be used for the determination of the ratio of the activity of substance i in a solution of molality mβ to the activity in a solution of molality mα. Here mβ and mα are the respective molalities observed at two positions xβ and xα within a two-component solution which has come to equilibrium in an ultracentrifuge. The calculation involves the densities of the solution at all positions between xβ and xα and the partial molal volumes of i for the two molalities mβ and mα. For each of these molalities partial volumes are required for the range of pressure between the equilibrium pressure acting on the solution of that molality and the pressure (ordinarily one atmosphere) of interest to the investigator. Some information concerning a solution containing more than two components can be derived from ultracentrifugal experiments if adequate density data are available.